A voltage controlled oscillator (VCO) or oscillator is a component that can be used to translate DC voltage into a radio frequency (RF) voltage or signal. In general, VCOs are designed to produce an oscillating signal of a particular frequency ‘f’ corresponding to a given tuning voltage. The frequency of the oscillating signal is dependent upon the magnitude of a tuning voltage Vtune applied to a tuning diode network across a resonator circuit. The frequency ‘f’ may be varied from fmin to fmax and these limits are referred as the tuning range or bandwidth of the VCO. The tuning sensitivity of the VCO is defined as the change in frequency over the tuning voltage and it is desirable to tune the VCO over a wide frequency range with a small tuning voltage range.
A high frequency signal can be generated either by an oscillator operating at a fundamental frequency or a harmonic oscillator. An oscillator operating at the fundamental frequency typically suffers from a low Q-factor, insufficient device gain and higher phase noise at a high frequency of operation. In contrast, harmonic oscillators may be operated at a lower frequency, and generally include a high Q-factor, high device gain and low phase noise. Harmonic oscillators, however, are generally more costly and typically employ YIG resonators to achieve their operational benefits.
The cascade structure and the parallel structure are the two configurations known for the harmonic oscillators. The cascade structure supports second-harmonic oscillation based on frequency-doubling. On the other hand, the parallel structure supports Nth harmonic frequency oscillations (N-push/push-push oscillator topology as a Nth harmonic oscillator) based on the coupled-oscillator approach. The frequency doubler and other means of up-conversion may provide a practical and quick solution to generate a high frequency signal from an oscillator operating at a lower frequency, however, they may also introduce distortions and provide poor phase noise performance.
The magnitude of the output signal from a VCO depends on the design of the VCO circuit and the frequency of operation is in part determined by a resonator that provides an input signal. Clock generation and clock recovery circuits typically use VCOs within a phase locked loop (PLL) to either generate a clock from an external reference or from an incoming data stream. VCOs are therefore often critical to the performance of PLLS. In turn, PLLs are essential components in communication networking as the generated clock signal is typically used to either transmit or recover the underlying service information so that the information can be used for its intended purpose. PLLs are particularly important in wireless networks as they enable the communications equipment to quickly lock-on to the carrier frequency onto which communications are transmitted.
The popularity of mobile telephones has renewed interest in and generated more attention in wireless architectures. This popularity has further spawned renewed interest in the design of low noise wideband oscillators. The recent explosive growth in the new families of cellular telephones and base stations using universal mobile telephone systems (UMTS) has stirred a need for developing an ultra-low noise oscillator with a fairly wide tuning range. The demands of wideband sources have generally increased telescopically because of the explosive growth of wireless communications. In particular, modern communication systems are typically multi-band and multi-mode, therefore requiring a wideband low noise source that preferably allows simultaneous access to DCS 1800, PCS 1900 and WCDMA (wideband code division multiple access) networks by a single wideband VCO.
The dynamic operating range and noise performance of a VCO may limit or affect the performance of the PLL itself, which in turn may affect the performance of the device in which the PLL is employed, e.g., RF transceivers, cell phone, modem card, etc. Broadband tunability of VCOs represents one of the more fundamental tradeoffs in the design of a VCO, impacting both the technology and the topology used. The dynamic time average quality factor (i.e., Q-factor) of the resonator, as well as the tuning diode noise contribution, affect the noise performance of a VCO. Furthermore, the dynamic loaded Q is, in general, inversely proportional to the operating frequency range of the VCO.
Despite the continuous improvement in VCO technology, low phase noise typically remains a bottleneck and poses a challenge to RF transceiver (transmitter—receivers) design. This is typically considered due to the more demanding parameters of the VCO design: low phase noise, low power consumption and wide frequency tuning range.
In LC-resonator based VCOS, phase noise and power consumption typically depend primarily on the time average loaded Q-factor of the resonator circuit and the non-linearities associated with the tuning network, which typically employ varactors. The frequency tuning range is determined by the usable capacitive tuning ratio of the varactor and parasitic associated with the tuning network because the parasitic deteriorates and limits the effective tuning capability of the varactor at a high frequency. As the loss-resistance of the tuning network (e.g., varactor and resonator) determines the quality factor, attention is usually paid to the resistive behavior. The frequency range over which a coupled resonator circuit can be tuned by means of the tuning diode depends on the useful capacitance ratio of the tuning diode and on the parallel and series capacitance present in the circuit.
As the frequency for wireless communication shifts to higher and higher frequency bands, generation of an ultra-low noise, wideband, thermally stable and compact signal source at a relatively low cost becomes more and more challenging due to the frequency limitations of the active devices and broadband tunability of the tuning diode. In the past, wide tuning range and good phase noise performance were generally considered to be opposing requirements due to the problem of the controlling the loop parameters and the dynamic loaded Q of the resonator over the range of wideband operation.
Typically, the phase noise of a microstrip line resonator-based wideband VCO is −80 dBc/Hz @10 KHz (kilo-hertz) for a frequency band of 1600–3200 MHz (mega hertz) operating at 15V (volts), 45 mA (milli-ampere). A YIG resonator based VCO offers wideband tunability with an external DC magnetic field, but at a high price. In addition, the YIG resonator based VCO is not amenable to integration in chip form.
Thus, there is a need for a wideband oscillator, preferably having octave-band tunability, that offers a cost-effective alternative to the YIG resonator based wideband VCO in the frequency range of the L (0.95–1.5 GHz), S (1.7–2.3 GHz), and C (4–6 GHz) bands.